Bringing Back the Brontosaurus

That got Haussler thinking. Scientists had reconstructed the sequences of individual genes from extinct species. But no one had even started working on re-creating an entire genome. Of course, the genomes wouldn't always line up - evolution re­arranges them over time. But the fragments could still be compared. And evolution tends to preserve exactly those parts that are most important.

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Here's an analogy: You ask 10 friends to remember the letter G. But the next day you discover that some, including you, have forgotten it. When you ask all 10 what the letter was, four say "G," while the others choose random letters. Since "G" is the most common response, you can pretty safely assume that G is the letter you told them. Do the same thing several billion times with the DNA sequences of mammals that exist today and you should be able to determine the genome of the common ancestor from which those mammals evolved. The more genomes you feed into the model, the more accurate your result will be.

One of Haussler's graduate students, Mathieu Blanchette, tested out the technique. Using a sequence of virtual DNA as complex as a real genome, he programmed his computer to make the sequence evolve in a way that mimicked nature. He then used the "descendants" to try to reconstruct the original genome. The results astonished Blanchette, who is now a professor at Montreal's McGill University. "It actually worked."

Haussler, Blanchette, and their ­collaborator, Webb Miller at Penn State, hope to release the program they've developed into the public domain later this year, allowing anyone to build the genomes of extinct animals. Haussler expects the reverse-evolution machine to "keep people busy for a long time."

Biologists can give you lots of reasons why ur-mammals won't roam the earth again anytime soon. For starters, genomes are really long. A typical mammalian genome contains billions of base pairs. Geneticists have no idea, at present, how to construct DNA sequences of such length and insert them into cells.

There's another big issue: mistakes. Haussler estimates that he could determine the ur-mammal genome with 98 percent accuracy. But of course there's no way to double-check without the original DNA. Plus, 2 percent is a lot. A human genome that was 98 percent correct would still contain 120 million errors, any of which could cause horrific problems.

The genomes of some extinct animals will be much harder to reconstruct than others. The ur-mammal has lots of present-day descendants, which is why Haussler chose it as his initial target, but dinosaurs don't. Reconstructing the genome of a Tyrannosaurus rex would therefore require inspired guesswork based on the genomes of related species like birds and turtles, as well as DNA fragments recovered from fossils. (And suddenly we're back in Jurassic Park.)

Then there are the unanticipated problems that come about when you fool around with nature. "There could be unforeseen interactions between an extinct species we bring back to life and ourselves," says Christos Ouzounis, an expert in computational genomics at the European Bioinformatics Institute in Cambridge, England. And even if we could re-create, say, a bronto­saurus, it would be plunked down in a place where it didn't belong and where it would have no adults to teach it how to be a proper brontosaurus.

Are any of these objections show­stoppers? Probably not. Biologists have already succeeded in reconstructing viruses - organisms so simple that whether they're alive is a matter of semantics. The next, much harder step will be to build microorganisms. While biologists need to know a lot more about how cells work to do that, they can already modify an existing microbe or virus to create an earlier version of that organism - scientists recently rebuilt a strain of the 1918 flu that killed more than 50 million people.

Resurrecting extinct species will be much more difficult, but the prospect now exists. Researchers continue to get better at extracting DNA from ­fossils, and Haussler's reverse-engineering technique will become commonplace as more genomes from modern-day organisms are sequenced. According to Miller, within the next couple of centuries humans should be able to make any creature they want.

For now, Haussler and his colleagues are focused on more immediate, though still ambitious, goals. They plan to explore the functions of ancient DNA segments by bioengineering them into mice, and they'd like to identify the specific genetic changes that transformed the ur-­mammal into an upright, hairless, big-brained primate. But in the long run, Haussler says, the potential is unlimited. "These are scientific opportunities that rarely come along in a person's lifetime."

Steve Olson (solson@comcast.net) is the author of Count Down: Six Kids Vie for Glory at the World's Toughest Math Competition.